This invention relates to a pneumatically operated pressure switch for monitoring the air pressure in a tire. The switch includes a housing capsule open towards the interior of the tire and is insertable into a corresponding opening of a vehicle rim. The housing capsule includes an electrically conducting first switching membrane exposed to the tire pressure; a reference pressure chamber filled with gas and sealed off from the tire by the first switching membrane; a first switching contact which can cooperate with the first switching membrane to provide a conductive closed path; and an oscillating circuit in series with the first switching contact and the first switching membrane.
The filling medium of a vehicle tire is exposed to substantial temperature fluctuations due to continually changing environmental conditions, friction and flexing forces. Pneumatically operated pressure switches of the above-described type which are used for monitoring a pressure or a pressure limiting value of a filling medium need to be provided with a temperature compensating means.
For this reason, DE-OS 28 32 447 provides a reference pressure chamber with such pressure switches. The pressure switch is located in a housing which is pneumatically isolated from the interior of the vehicle tire by means of a membrane. The membrane cooperates with a switching contact insulatingly carried in the housing, and protrudes into the center of the reference pressure chamber with respect to the switching contact. The cooperation of the membrane and the switching contact serves to connect or close a signal transmitting circuit. The reference pressure chamber is filled with pressurized gas. The pressure of the gas is chosen such that the membrane, when the correct pressure of the filling medium of the vehicle tire is provided, rests against the switching contact and lifts up as the pressure of the filling medium decreases. Since the reference pressure chamber and the vehicle tire are subjected to the same temperature on opposite sides of the membrane, the membrane switch is temperature balanced.
However, a disadvantage with such pressure switches is that the sealing capacity of the reference pressure chamber, and as a result the stability of the pressure of the gas over a long period of time such as, for example, the normal life expectancy of a vehicle, is not guaranteed. Because of the unavoidable leakages which can occur due to, for example, material weakening or gas diffusion, this pressure may drop substantially below its nominal value even after a relatively short operational period. An inaccurate tire pressure will no longer be noticeable and may lead to dangerous situations during the operation of the vehicle since the driver relies upon the system. Furthermore, it is very difficult to test the correct functioning of the pressure switch since the pressure in the reference pressure chamber cannot be directly measured.
It is therefore an objective of the present invention to provide a pneumatically operated pressure switch for monitoring the tire pressure which is itself monitored continuously as to its correct functioning during the operation of the vehicle, and yet is simple in construction, and capable of being produced cost-efficiently.
These and other objectives are attained by the present invention by providing a pneumatically operated pressure switch in the housing capsule with a second switching means for sensing a change in the reference pressure chamber and which is connected to the oscillating circuit for changing the oscillating circuit upon sensing a change of gas pressure in the reference pressure chamber.
The advantages provided by preferred embodiment of the present invention are primarily that a pressure switch for monitoring the tire pressure is provided which, during the operation of a vehicle equipped with the invention, is monitored continuously regarding a correct pressure in its reference pressure chamber and thereby its functional accuracy. Furthermore, the invention is of simple constructional design and as a result is sturdy and can be manufactured both easily and cost-efficiently.
Further objects, features and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings, which show for purposes of illustration only, embodiments constructed in accordance with the present invention.